Process of precipitating substantially anhydrous calcium chromate and product resulting therefrom



United States Patent PROCESS OF PRECIPITATING SUBSTANTIALLY ANHYDROUS CALCIUM CHROMATE AND 2,745,764 Patented May 15, 1956 2 in the needle-like or rod-form crystals. These rod or needle-form crystals have high crushing resistance and are highly inert to rehydration and even to calcination at 1000-1200 F. The substantially anhydrous calcium 5 chromate product has a low bulking density of'9 to 45' PRODUCT RESULTING THEREFROM pounds per cubic foot, as compared to 50 to 55 pounds Holbert E. Dunn, Grafton, and Ellis J. OBrien, Pittsper cubic foot for atmospheric precipitated crypto-crystalburgh, Pa., assignors to Vanadium Corporation of line or rhombic dipyramidal material'madeaccording to America, New York, 'l of Delaware prior known processes. Our product also is to be contrasted with calcium chromate produced by autoclaving at No g 3 ch 1953 3 atmospheres pressure which has a bulking density of 87 pounds per cubic foot and is of the rhombic dipyra- 11 Claims. (Cl. 106-302) midal form. The absolute specific gravity of the individual crystals of our calcium chromate product is from This invention relates to a process of precipitating 15 2- 0 to 3.10. This calcium chromate product having substantially anhydrous calcium chromate from sodium y 1/3 to 1/6 the hulking density of P known P P chromate solutions and to the product resulting therefrom. tilted Calcium chromate, y be distinguished as a The sodium chromate solutions may result from roasting grade, as contrasted to the regular yand/or leaching chrome ores, slags, pulverized ferroour Process y he Carried out in the following chremium or other chromium bearing materials. The t, the quantities being y Weight unless Otherwise process relates more particularly to improvements in the speelfieally Statedprecipitation of substantially anhydrouscalcium chrop 1-S0dh1m silicate solution s added t a S0di1lm mate b treating Sodium h m Solutions i h l i chromate solution. The sodium silicate solution is added chlorida slowly so as to prevent hydrolysis of the sodium silicate According to a known Process, a l i hl id to free silicic acid, which would not react with the sodium solution is added to a sodium chromate solution and the illuminate IleXt added to form sodium alumino-silieate mixed solution is heated to precipitate calciumchromate. 'y P Catalyst, but Would pp 011 the surface The reaction may be represented by the following equa- 0f the slurry as a gummy Seum- The sodium chromate tion; solution containing sodium'silicate'is hereinafter referred to as Solution 1. (1) Nazcroll"CaChZZNaCI+CaCIO4 Step 2.-A solution of aluminum sulphate and caustic When the reaction is carried out at atmospheric pressoda (or its equivalent, sodium aluminate), hereinafter sure, and completed at a pH of about 6.5-7.0, the product referred to as Solution 2, is then added to Solution 1. obtained is in the form of the dihydrate CaCrO4.2H2O The resulting slurry is maintained at a temperature of comprising 81.25% CaCrOe and 18.75% H2O,- in yellow 70l90 F. and at a pH of 7.2-12 during a conditioning monoclinic prismatic or rhombic crystal form. The diperiod for a time sufiicient to form nuclei of sodium hydrate is soluble in water to the extentof 14.0 to 20.3 aluminosilicate; This conditioning period will vary ac grams per 100 grams of water at 20 C. Because of its cording to the'temperature of the slurry. The object of solubility and highwater content, this chromate is'not the conditioning period is to form nuclei of sodium Suitable for direct application inceramicandpaint pig- 40 aluminosilicate fromthe sodium silicate and sodium ments or as apyrometallurgical oxidant. aluminate; If the slurry isat a temperature of 70 F., Calcium chromate monohydrate CaCrOsJ-IzO can be this conditio'ning period may'require about 15 minutes. made by mixing a sodium chromatersolmion i i If it is at a temperature of 120" F, it may require 10 say, 75.5 grams per liter chromium-with calcium chloride minutes; if at a temperature of 3 Period 0f 5 to the extent of an excess of 20 to of theoretical 45 minutes; n if at a temperature of a p requirements, and heating the mixed solution to boiling. 2 03 minutes. The pH of the slurry during this condi- Th product i i rhombic crystalline f containing tioningperiod must be at least 712 and'may be as highas 90.3% calcium chromate and 9 7% H2O; whichv corre- 12.0, the preferred PH being 7.4-8.2, and the preferred sponds closely to the monohydrate CaCrOLHzO. Howtemperature being 120190 F. ever, only about 61% f th h omi v t t can be 0 When Solution 2' is added to Solution. 1, the cloudiness precipitated by this method in which the reaction is carried fir in s n n decreases during the conditioning t d completed at s I-I f about 6,5 to 7 period; The decrease in cloudiness during the condition- The present invention provides various improvements lhg Pertocl indicates fonhatien Sodium alum n siliin the processes and products heretofore known, cate nuclei, but the solution Still remains cloudy during The calcium chromate product'produced inaccordance the Conditioning P The reeetloh fer forming the with this inventi i substantially anhydrous dlh a sodium aluminosilicate nuclei is represented by Equation low bulking density. It is in the form of an intimate 2 s fbllOWSI Albite feldspar (2) 3NagSi O9 QAIKSOJX! 10N80H=2(Nu O.A .6SiOz) 6N83SO4 5Hg0 mixture of slender needle-like or'rod-form calcium chro- Step 3.-After the nuclei, of sodium alu i o ilj ate mate crystals, usually of a size order oi S-to 15 microns have formed u g the Conditioning Period, 3501115011 of in diameter by 50 to mi ro in le th, a d a ll calcium chloride is mixed with the sodium chromate soluamoum (usually 25 to f separate, discreteQgJfition, the calcium chloride amounting to at least 120%. phous, milky particles of a odiu al i ili t gel 65 of the stoichiometric requirements of calcium chloride Such as albite feldspar having the composition for precipitating all of the chromium as calcium chromate. The calcium chloride solution is reheated to a Nazo'Alzos'lssloz temperature of 120-190 F. before it is I I IlXCd with the or lime-soda feldspar, namely, a combination of.thissalbite sodium chromate solution, the preferred temperature of with anorthite, 2(CaO.AlzO3;2SiOz). These feldspathic mixing these'solutions being about F; The addicompositions act as crystal-shaping catalyst or nuclei. to

insure the formation of the calcium chromate. preciptate.

tion of the preheated calcium: chloride solution to the sodium chromate solution causes precipitation of a large was 7.8.

proportioniofthe chromium as calcium chromate, the

reaction being represented by Equation 1.

If the solutions are mixed at temperature higher than about 190 F., the rate of calcium chromate precipitation is so rapid thatthe slurry becomes so thick that agitation of it becomes difficult. .Also the absorption of ,NaCl 'in the calcium chromate product increases rapidly at temperatures above 190 E, requiring increased water washing of the precipitate and increasing the The precipitation of large crystals chloride solution is mixed with the sodium chromate solution at a temperature belowabout 120 F, the precipitation is so slow as to be economically unfeasible.

If the calcium chloride solution is mixed with the sodium chromate solution at a temperature of about 160 F., precipitation of approximately 80% of the I chromium content as calcium chromate occurs in about.

7 1 minute.

The reaction of sodium silicate, aluminum sulphate and sodium hydroxideto form sodium aluminosilicate, as represented by Equation 2, is sluggish. Likewise, the

reaction of sodium silicate with sodium aluminate to form sodium aluminosilicate, is sluggish. The formation of calcium chromate from sodium chromate and calcium' chloride, as represented by Equation 1, is very rapid.

The solution was heated to 120 F. and 34.5 kgs. of 40 Baum sodium silicate (28.8% SiOz by weight) were added slowly to the solution. After about 75% of the sodium silicate had been added, the solution became cloudy. The sodium chromate solution containing sodium silicate is hereafter referred to as Solution 1.

10 kgs. of aluminum sulphate, A12(SO4)3.18H2O, technical grade, and 10 kgs. caustic soda flake (76% Nazi?) were dissolved in water, and the solution diluted in 200 liters with water. This formed sodium aluminate cc .taining excess caustic soda. This solution is hereinafter referred to as Solution 2.

A solution was made of 340 kgs. calcium chloride, technical grade, 7477% CaClz, 1000 liters water. i

This solution is referred to hereinafter as oolution 3.

Solution 2 was added to Solution 1 with agitation,

' resulting in a pH of 11.2 and a temperature of about Heat was then applied to bring the cloudy solution to 160 Fin about 5 minutes, this step being the conditioning period for forming nuclei of sodium aluminosilicate.

Solution 3 was heated to 160 n. and, while stirring,

' it was added to the mixture of Solutions 1 and 2 in a period of about 1 minute, causing the precipitation of approximately 80% of the chromium content as calcium chromate in that period. .This addition of the calcium chloride solution lowered the pH of the slurry to about 8.7.

Accordingly, it is necessary to employ the conditioning period for forming the nuclei of sodium aluminosilicate prior to the addition of the calcium chloride solution to the sodiurn chromate solution. When this conditioning period is employed, precipitation of amorphous albite feldspar gel occurs according to Reaction 2 to insure the formation of the calcium chromate precipitate in the needle-like or rod-form crystals which are characteristic of the low bulking density calcium chromate product.

Step 4.The slurry is then heated to a higher temperature in the neighborhood of the boiling point, which is about 215 F. Preferably the heating is carried out at a temperature of'about 2102l5 F. During pre cipitation of the calcium chromate, the pH of the slurry becomes lower. Samples of the slurry are analyzed from time to time and if its pH falls below 8.2, it is adjusted to this value by adding caustic soda solution and then heating is continued until precipitation is completed.

Step 5,Thereafter the slurry is separated, for example, by filtration, and then is washed and dried.

In place of using a calcium chloride solution, We may use calcium chloride in solid form.

The following are specific examples of the manner in which our invention may be carried out.

Example 1 In this example we started with commercial grade sodium bichromate crystals, Na2Cr2Om2H2O, and made a sodium chromate solution from it by adding caustic soda. This sodium bichromate was dehydrated so'that it contained 37.85% chromium.

205 kgs. of anhydrous sodium bichromate containing.

The pH of the adjusted solution a with other briquetting aggregates.

The slurry was then heated to 210-215 F. in a period of 20 minutes and maintained at that temperature for 15 minutes.

The hot slurry was filtered on a suction filter to give a cake of 70% moisture, which was then washed with 2 /2 displacement washes with hot water F.) to

give 867.3 kgs. .of'wet filter cake at 70% moisture, or 358 kgs. when dried to 27% moisture, ordinarily found desirable for incorporating the calcium chromate product The recovery of chromium in the product amounted to 94%. A sample of the product dried at 230 F. to equilibrium moisture content had a bulking density of 13.95 pounds per cubic foot and analyzed as follows: 7

85.89% CaCIOet 2.88% SiOz In filtering theprecipitated slurry, 1385 liters of strong filtrate containing 0.85 kg. of chromium, and 1538 liters of weak wash filtratecontaining 4.34 kgs. of chromium, were recovered. The strong filtrate was discarded primarily because of its low chromium concentration, but the weak wash filtrate could be reused in solution make-up instead of Water; 7 This practice would permit 2 to 4.5% additional chromium recovery.

Micro-examination of this calcium chromate product showed it to be a mixture of crystalline, needle-form calcium chromate and separate, discrete, amorphous, milky particles of sodium aluminosilicate gel. The product does not. have good bonding power when it is briquetted by itself or used in a briquetting mix with other non-binding aggregates. However, we have found that the bonding power of the calcium chromate product can be improved by adding a gel hardener such as sodium or potassium dichromate or sodium chloride at the time the calcium chromate product is incorporated in the briquetting mix, or simply by permitting say 1% or less sodium chloride to remain residual in the final calcium chromate product by limiting the amount of Water wash-.

CO2 should be used so that there will not be much CO2 available for'rea'cting Withthe calcium chloride.

Example 2 Sodium chromate liquor was obtained by water-leaching a soda-lime roasted chrome ore, at a pulp density of 42.4% solids by weight, the sodium chromate liquor having a pl-I of 11.7 and analyzing:

74.00 grams/liter chromium 0.12 gram/liter SiO2 1.56 grams/liter A1203 910 liters of this solution was heated to 80 F. and 80 liters of 1:1 hydrochloric acid (40 liters of 38% HCl plus Water to 80 liters) was aded to give a pH of 6.9 and a heavy gelatinous precipitate. After boiling for 3 minutes this precipitate was filtered ofi and discarded. The pH was then adjusted to 8.0 by the addiiton of kgs. of flake caustic sodaananren readjusted to 7.6 by addition of 3 liters of 1:1 hydrochloric acid.

This solution was their heated to 120 F. and 29.95 kgs. of 40 B. sodium silicate (28.8% SiOz by weight) was added slowly to give Solution 1.

173 liters of sodium aluminate (Solution 2) was then added to Solution 1, With agitation for a conditioning period of 10 minutes. Heat was then applied to bring the cloudy solution to 160% F. in about 5 minutes, whereupon Solution 3 (295 kgs. of calcium chloride in 1000 liters of water) preheated to 160 F., was added to the mixture of Solutions 1 and 2, while stirring, in a period of about 1 minute. Then heating was continued with stirring to 210 F. in 25 minutes, holding for minutes longer at temperature, with the pH of the slurry showing 8.9 upon reaching 210 F. The slurry was then filtered and washed to obtain a recovery efiiciency of 87.6% of the total chromium and a pH of the filtrate of 8.2. The recovered product, dried at 230 F., analyzed as follows:

84.18% CaCrO4=28.06% Cr 4.29% SiOz 2.00% ignition loss at 1000 F.

Micro-examination showed the product to consist of a mixture of fine granular dipyrarnidal and coarse prismatic needles of yellow calcium chromate with fine granular discrete grains of milky white binder.

The product had a bulking density of 34.3 pounds per cubic foot.

Example 5 Sodium chromate liquor was obtained by leaching a 1650 F. roast of ground ferrochroinium with lime and salt in sodium carbonate solution at a pulp density of 22.3 solids, to give a sodium chromate liquor at a pH of 11.6, analyzing:

75.94 grams/liter chromium 0.30 gram/liter SiOz 0.34 gram/liter A1203 820 liters of this solution was heated to 74 F. and 75 liters of 1:1 hydrochloric acid was added to give a pH of 6.9 and a very light fiocculent precipitate, which was not removed.

The pH was then readjusted to 7.7 by the addition of 7.0 kgs. of flake caustic soda, and heated to 120 F., whereupon 27.6 kgs. of 40 B. sodium silicate was added slowly to give Solution 1.

161 liters of sodium aluminate (Solution 2) was then added to Solution 1 with agitation for a conditioning period of 10 minutes.

The cloudy mixture of Solutions 1 and 2 was heated with agitation to 160 F. in about 5 minutes, and Solution 3 (273 kgs. of calcium chloride in 900 liters of water) previously heated to 160 F., was added to the mixture of Solutions 1 an-d2',- while stirring, in a period of about 1 minute,- and heating was continued with stirring to 210 F. in 20 minutes, holding 15 minutes longer at temperature. The slurry had a pH of 8.8 upon reaching 210 F.

The slurry was filtered and washed to obtain a recovery efliciency of 86.6%" of the total chromium, and a pH of the filtrate of 8.2. The recovered product, dried at 230 F., analyzed as follows:

81.57% CaCrO4=27.19% Cr 4.10% SiOz 0.30% NazO 0.01% 01 4.00 ignition loss at 1000" F.

This applicationis related to our copeu'ding applications Serial Nos. 340,401, 340,402 and 340,404, all filed March 4, 1953.

The invention is not limited to the preferred embodiment but may be otherwise embodied or practiced within the scope of the following claims.

We claim:

1. The process of precipitating a substantially anhydrous calcium chromate product, which comprises adding sodium silicate solution to a sodium chromate solution, adding sodium aluminate solution to the sodium chromate solution, maintaining the slurry at a temperature of -190 F. during a conditioning period for a time sutiicient to form nuclei of sodium aluminosilicate and at a pH of 7.212, adding calcium chloride in an amount of at least of the stoichiometric requirements of calcium chloride for precipitating calcium chromate, and heating the resulting slurry to a temperature of ZOO-215 F. to precipitate a mixture or" substantially anhydrous calcium chromate and sodium aluminosilicate gel.

2. A process according to claim 1, wherein the slurry is maintained during the conditioning period at a pH between 7.4 and 8.2.

3. A process according to claim 1, wherein the slurry is maintained during the conditioning period at a temperature between 120 and F.

4. A process according to claim 1, wherein the slurry.

is maintained during the conditioning period at a pH between 7.4 and 8.2 and at a temperature between 120 and 190 F.

5. The process of precipitating a substantially anhydrous calcium chromate product, which comprises adding sodium silicate solution to a sodium chromate solution. adding sodium aluminate solution to the sodium chromate solution, maintaining the slurry at a temperature of 70-190" F. during a conditioning period of at least 2 minutes to form nuclei of sodium aluminosilicate and at a pH of 7.2-12.0, adding calcium chloride in an amount of at least 120% of the stoichiometric requirements of calcium chloride for precipitating calcium chromate, and heating the resulting slurry to a temperature of 200215 F. to precipitate a mixture of substantially anhydrous calcium chromate and sodium aluminosilicate gel.

6. The process of precipitating a substantially anhydrous calcium chromate product, which comprises adding sodium silicate solution to a sodium chromate solution, adding sodium aluminate solution to the sodium chroma te solution, maintaining the slurry at a tempera-' ture of 70190 F. during a conditioning period for a tion in an amount of at least 120% of the stoichiometric requirements of calcium chloride for precipitating calto precipitate, when calcium chloride is added to it and the solution is heated, a substantially anhydrous calcium chromate product of low bulking density which is a mixture of crystalline, needle-form calcium chromate and separate, discrete, amorphous, milky particles of sodium aluminosilicate gel, which improvement comprises adding sodium silicate solution and sodium aluminate solu-' tion to the sodium chromate solution, and maintaining the solution at a temperature of 70190 F. and at a pH of 7.2-1.2.0 during a conditioning period for a time sufficient to form nuclei of sodium aluminosilicate.

, V 9. A process according to claim 8, wherein the sodium time sufficient to form nuclei of sodium aluminosilicate I p and at a pH of 7.2-12.0, adding a calcium chloride solu- 8 silicate solution and sodium aluminate' solution are in amounts to give 23-45% SiOz and 0.65- 1.2%. A1203 by weight in the final precipitated calcium chromate product.

10. A substantially anhydrous calcium chromate'product having a bulking density of 9 to 45 pounds, per cubic foot, and being an intimate mixture of'crystalline, needleform calcium chromate and separate, discrete, amorphous, milky particles of sodium aluminosilicate gel. 7 i

11. A calcium chromate product according to claim 10, wherein the sodium aluminosilicate gel is of the approximate molecular formula Na2O.Al2O3.6SiO2 and amounts to 25-65% by Weight of the calcium chromate product.

References Cited in the file of this patent UNITED STATES PATENTS 1,660,696 Wallin Feb. 23,1928 2,276,314 Kirk Mar. 17, .1942 2,276,315 Kirk Mar. 17,-; 1942 2,395,472 Fernald Feb; 26, 1946 2,493,789

Udy Ian. 10, 1950 OTHER REFERENCES Chemical Process Industries (Shreve);- pub. by Mc- Graw-Hill Book Co., Inc., N. 'Y., N. Y. (1945); page 498 is relied upon. I

Barrer et al.: I. of chemosociety, May 1952, pp. 

5. THE PROCESS OF PRECIPITATING A SUBSTANTIALLY ANHYDROUS CALCIUM CHROMATE PRODUCT, WHICH COMPRISES ADDING SODIUM SILICATE SOLUTION TO A SODIUM CHROMATE SOLUTION, ADDING SODIUM ALUMINATE SOLUTION TO THE SODIUM CHROMATE SOLUTION, MAINTAINING THE SLURRY AT A TEMPERATURE OF 70-190*F. DURING A CONDITIONING PERIOD OF AT LEAST 2 MINUTES TO FORM NUCLEI OF SODIUM ALUMINOSILICATE AND AT A PH OF 7.2-12.0, ADDING CALCIUM CHLORIDE IN AN AMOUNT OF AT LEAST 120% OF THE STOICHIOMETRIC REQUIREMENTS OF CALCIUM CHLORIDE FOR PRECIPITATING CALCUM CHROMATE, AND HEATING THE RESULTING SLURRY TO A TEMPERATURE OF 200-215* F. TO PRECIPITATE A MIXTURE OF SUBSTANTIALLY ANHYDROUS CALCIUM CHROMATE AND SODIUM ALUMINOSILICATE GEL. 